The problems associated with techniques known in the coating and finishing industry for the application of a coating to a substrate have been detailed in U.S. Pat. No. 4,247,581 to Cobbs, Jr., et al. Reference to such patent should be made for a detailed treatment of that subject, but it may be briefly stated for purposes of the present invention that the coating and finishing industry, despite extensive research and development effort, remains predominantly dependent on the use of solvent-containing coatings. It has been suggested that possibly the most serious concern of the industry today, considering both raw material usage and problems with environmental effects, concerns the solvent components of paint. In a spray coating application of a resinous material, the resinous material is typically dissolved in an organic solvent to provide a viscosity suitable for spraying This is required because it has been found that at each stage of the process for atomizing and conveying a resinous material in liquid form to a substrate, the liquid resists high speed deformation. Organic solvents are added to the resinous liquid because they have the effect of separating the molecules of resinous material and facilitating their relative movement making the solution more deformable at high speeds and therefore more susceptible to atomization. While a great deal of effort has been expended to reduce the volume of liquid solvent components in preparing high solids coating compositions containing above 50% by volume of polymeric and pigmentary solids, little success has been achieved. Most high solids coating compositions still contain from 15-40% by volume of liquid solvent components.
The problem with such a high volume content of liquid solvents is that during handling, atomization or deposition of solvent coating compositions, the solvents will escape and can become air contaminants if not properly trapped. Moreover, once a solvent coating is applied to a substrate the solvents are susceptible to escape from the film by evaporation and such evaporated solvents also contaminate the surrounding atmosphere. In addition, since most solvents react with oxidants, pollution problems of toxicity, odor and smog may also be created. Generally, attempts at overcoming such environmental problems are costly and relatively inefficient.
The invention disclosed in the '581 patent mentioned above is a method and apparatus for atomizing and conveying high solids paint or other film forming solids to a substrate for coating. In accordance with the method disclosed in the '581 patent, polymeric compositions having little or no solvent content and viscosities in the range of 300-30,000 centipoises are first foamed to a relatively stable energized state and thereafter subjected to an atomizing force for uniform coating of a substrate. An important element of the '581 invention is the recognition that the use of an energized, relatively stable foam in coating applications eliminates many of the major problems existing in the finishing and coating industry including a drastic reduction or elimination of the use of organic solvents. In contrast to conventional spraying or coating systems wherein foam was suppressed during the manufacture, pigmenting, tinting, and application of paint or coating materials to a substrate, it was found as disclosed in the '581 patent that an exceptional surface coating could be achieved by first converting a film forming solid into a foamed state and then disintegrating or atomizing the foam using known techniques.
Given the teachings of the '581 patent that it is desirable to convert high solid paint or other film forming solids to a foam prior to atomization and conveyance to a substrate, it should be stated that there are basically two generally used techniques to convert a liquid into a foam material. As discussed in U.S. Pat. No. 4,059,714 to Scholl, et al, for example, one method of producing a foam material from liquid involves the injection of air or a gas such as nitrogen into the liquid under a suitable pressure. The gas is forced into solution with the liquid and when the liquid-gas solution is subsequently dispensed into atmospheric pressure, the gas comes out of solution and becomes entrapped in the liquid to form a closed cell solid foam. The gas essentially evolves from the solution under atmospheric pressure in the form of small bubbles, causing the surrounding liquid to expand volumetrically. The result is a homogeneous solid foam having closed air or gas cells evenly distributed throughout. In the alternative, as disclosed for example in the '581 patent discussed above, a so-called blowing agent may be placed into solution with a liquid under appropriate temperature and pressure conditions. When the solution is exposed to a pressure less than that required to maintain the blowing agent in solution, gas bubbles are produced and trapped in the liquid, forming a closed cell solid foam.
Several parameters must be observed in the process of creating and/or atomizing the foam to achieve an even coating of high solids paint or a similar film forming material such as hot melt adhesives on a substrate. The subject invention is primarily directed to improvements in the nozzles utilized to generate and atomize the closed cell solid foam. The problem is one of creating a stable, low density foam which can be atomized prior to contact with the substrate. It was found using conventional paint or fluid nozzles that the point of formation of the high solids paint foam could not be adequately controlled. As mentioned above, whether a pressurized gas or a blowing agent is introduced into solution with a liquid to be foamed, the foaming will not occur until the solution is exposed to a pressure less than the maintenance pressure of the solution. Experiments with conventional fluid nozzles have shown that foaming of a solution containing high solids paint often occurred several inches from the end of the nozzle. As is well known, air jets or similar means are typically disposed immediately adjacent the end of foam nozzles to disintegrate or atomize the foam prior to contact with the substrate. This assures even coating of the substrate. It can be appreciated that if foaming does not occur immediately at the point of exit from the nozzle, then complete atomization of the foam is impossible. The result is uneven coating of even coating of the substrate. It can be appreciated that if foaming does not occur immediately at the point of exit from the nozzle, then complete atomization of the foam is impossible. The result is uneven coating of the substrate, running or dripping of the paint and similar problems.
In addition to the problem of forming the foam at the proper time, prior art foam nozzles tend to cause sputtering or non-uniform flow of the stream of foam formed from a high solids coating or similar material. It has been found that such a problem creates the same undesirable results as incomplete atomization of the foam discussed above.
It is therefore an objective of this invention to provide a nozzle assembly for converting a liquid containing gas maintained under pressure in the liquid to a foam for application to a substrate.
It is another object herein to provide a nozzle assembly capable of producing complete foaming of a solution containing a liquid and a gas maintained under pressure in the liquid, prior to the discharge of the liquid from the nozzle assembly, to assure complete atomization upon discharge.